Ceramic tubes have found a use in the manufacture of Solid Oxide Fuel Cells (SOFCs). There are several types of fuel cells, each offering a different mechanism of converting fuel and air to produce electricity without combustion. In SOFCs, the barrier layer (the “electrolyte”) between the fuel and the air is a ceramic layer, which allows oxygen atoms to migrate through the layer to complete a chemical reaction. Because ceramic is a poor conductor of oxygen atoms at room temperature, the fuel cell is operated at 700° C. to 1000° C., and the ceramic layer is made as thin as possible.
Early tubular SOFCs were produced by the Westinghouse Corporation using long, fairly large diameter, extruded tubes of zirconia ceramic. Typical tube lengths were several feet long, with tube diameters ranging from ¼ inch to ½ inch. A complete structure for a fuel cell typically contained roughly ten tubes. Over time, researchers and industry groups settled on a formula for the zirconia ceramic which contains 8 mol % Y2O3, and is referred to as yttria stabilized zirconia (YSZ). This material is made by, among others, Tosoh of Japan as product TZ-8Y.
Another method of making SOFCs makes use of flat plates of zirconia, stacked together with other anodes and cathodes, to achieve the fuel cell structure. Compared to the tall, narrow devices envisioned by Westinghouse, these flat plate structures can be cube shaped, 6 to 8 inches on an edge, with a clamping mechanism to hold the entire stack together.
A still newer method envisions using larger quantities of small diameter tubes having very thin walls. The use of thin walled ceramic is important in SOFCs because the transfer rate of oxygen ions is limited by distance and temperature. If a thinner layer of zirconia is used, the final device can be operated at a lower temperature while maintaining the same efficiency. Literature describes the need to make ceramic tubes at 150 μm or less wall thickness.
An SOFC tube is useful as a gas container only. To work it must be used inside a larger air container. This is bulky. A key challenge of using tubes is that you must apply both heat and air to the outside of the tube; air to provide the O2 for the reaction, and heat to accelerate the reaction. Usually, the heat would be applied by burning fuel, so instead of applying air with 20% O2 (typical), the air is actually partially reduced (partially burned to provide the heat) and this lowers the driving potential of the cell.
An SOFC tube is also limited in its scalability. To achieve greater kV output, more tubes must be added. Each tube is a single electrolyte layer, such that increases are bulky. The solid electrolyte tube technology is further limited in terms of achievable electrolyte thinness. A thinner electrolyte is more efficient. Electrolyte thickness of 2 μm or even 1 μm would be optimal for high power, but is very difficult to achieve in solid electrolyte tubes. It is noted that a single fuel cell area produces about 0.5 to 1 volt (this is inherent due to the driving force of the chemical reaction, in the same way that a battery gives off 1.2 volts), but the current, and therefore the power, depend on several factors. Higher current will result from factors that make more oxygen ions migrate across the electrolyte in a given time. These factors are higher temperature, thinner electrolyte, and larger area.
Fuel utilization is a component of the overall efficiency of the fuel cell. Fuel utilization is a term that can describe the percent of fuel that is converted into electricity. For example, a fuel cell may only convert 50% of its fuel into electricity, with the other 50% exiting the cell un-used. Ideally, the fuel utilization of a fuel cell would be 100%, so that no fuel is wasted. Practically, however, total efficiency would be less than 100%, even if fuel utilization was 100%, because of various other inefficiencies and system losses.
A challenge for fuel utilization at the anode is to move molecules of fuel into the pores of the anode. Another challenge is to move the waste products, i.e., water and CO2 molecules, out of the pores of the anode. If the pores are too small, then the flow of fuel inward and waste-products outward will be too slow to allow high fuel utilization.
An analogous condition exists for the cathode. Because air is only 20% oxygen, and has 80% nitrogen, there is a challenge to move oxygen into the pores and N2 out of the pores. Collectively, utilization of the fuel and air input to the device may be referred to as “gas utilization.”
One problem for gas utilization is that air and fuel can pass through the flow paths past the porous anodes and cathodes without the molecules ever entering the pores. The “path of least resistance” would lead a molecule to bypass the most important part of the fuel cell.
Additionally, if the gas molecules can't get into and out of the anode and cathode, then the fuel cell will not achieve its maximum power. A lack of fuel or oxygen at the anodes or cathodes essentially means that the fuel cell is starved for chemical energy. If the anode and/or cathode are starved for chemicals, less power will be generated per unit area (cm2). This lower power per unit area gives lower total system power.
In a tubular fuel cell device, such as that shown in FIG. 1 where the anode lines the inside of the tube and the cathode forms the outer surface with the electrolyte therebetween, it is wishful thinking to expect high utilization of fuel. The inside diameter of the tube, which forms the fuel passage, is very large when compared to the thickness of the anode. Anode thicknesses may be on the order of 50-500 nm, whereas tube diameters may be on the order of 4-20 mm. Thus, there is a high likelihood of fuel molecules passing through the large fuel passage without ever entering the pores of the anode. An alternate geometry for the tube is to have the anode on the outside of the tube. In that case, the problem could be worse because the fuel is contained within the furnace volume, which is even larger than the volume within the tube.
Within a multilayer SOFC, such as the Fuel Cell Stick™ device 10 depicted in FIG. 2 and developed by the present inventors, fuel utilization can be higher because the flow path for the gas can be smaller. FIG. 2 is identical to FIG. 1 of U.S. Pat. No. 7,838,137, the description of which is incorporated by reference herein. Device 10 includes a fuel inlet 12 feeding a fuel passage 14 to a fuel outlet 10, and an oxidizer inlet 18 feeding an oxidizer passage 20 to an oxidizer outlet 12. An anode 24 is adjacent the fuel passage 14 and a cathode 26 is adjacent the oxidizer passage 20, with an electrolyte 28 therebetween. By way of example, both the anodes 24 and fuel passages 14 can be made to a thickness of 50 nm, and this similarity in thickness, where the ratio of thickness can be near 1:1 (or a bit higher or lower, such as 2:1 or 1:2) can give a more optimal chance of molecule flow into and out of pores.
However, as the electrolyte is made thinner, such that the power per cm2 (W/cm2) goes up (or as the other elements of the structure are optimized to give higher power per area), the production of waste H2O and CO2 within the pores will increase. So, as power per area and volume increases, there is an increased need to exchange the gases in the porous structure more quickly.
Therefore, there is a need to better direct the gases into the pores and to flush waste products out of the pores. Higher utilization and/or better flow through the pores will give better system performance.